P0303 Fault Code
You can find information about the P0303 fault code on this page.
It is one of the most popular fault codes that people search for on this website (in the top 50).
We have given it a seriousness of 8/10 with 1 being the least serious and 10 being the worst.
Looking To Fix This Yourself?
By using a workshop/repair manual you could diagnose and even fix this issue yourself.
You can find a collection of free manuals containing loads of information specific to your car on our partner site.View Workshop/Repair/Service Manuals »
Fault code P0303 is the code is logged when the engine control unit (ECU) has detected a malfunction with cylinder 3. This occurs specifically when a misfire has been detected.
This code is a part of several diagnostic codes that inform of a cylinder that has misfired or is still misfiring.
These codes start with P0300, which represents multiple random misfires, and go up to the number of cylinders found within the vehicle. For example, a vehicle with a V10 engine will have P0310.
There are several reasons why a cylinder may misfire. The result is the misfiring cylinder is not producing sufficient power to operate properly. If a cylinder does not have enough fuel, air, or heat, a misfire can occur.
All vehicles can suffer a misfire but some of the most common engines to have these issues are those from Jeep and other General Motors brands, Ford, and Mazda.
Other Signs of This Problem
A misfire typically comes with several noticeable indicators that can inform you of the issue. These all relate to the sound and feel of the engine while idling or during acceleration.
It is possible to hear a misfire due to the absence of cylinder combustion. While this takes experience, the engine can also simply sound strange.
Other more noticeable signs will be the Check Engine light being illuminated, as well as rough idling and acceleration. The vehicle may also have trouble starting.
A misfiring cylinder can also come with decreased fuel economy as fuel is being used without harnessing any power from it.
What Could Be The Problem?
With any combustion-related issue, the cause could stem from an issue with the heat, fuel, or air supply to the cylinder. The computers that control the air-fuel mixture can also be at fault.
The list of possible causes is extensive as all three systems could be at fault, as well as downstream components.
A problem specific to petrol engines is an issue with the spark plug in question. If the spark plug or ignition coil is faulty, it will not provide enough heat to the cylinder to spark combustion. This causes a misfire even though there is the correct air-fuel ratio present.
Issues with the fuel injector for cylinder 3 can also cause a misfire as there is a lack of fuel present.
Vacuum leaks are another possible cause of a misfire. If a leak is causing a disruption in the ideal air-fuel ratio, it can cause a misfire.
In more serious situations, timing may be the culprit. This could be indicated by a code like P0011.
Fixes for a misfire depend on the system that is faulty. Depending on which system is causing the issue, the fix can be easy or complicated.
In a petrol engine, check the spark plug and ignition coil in question and compare them to a fresh one. If they are damaged, replacing them should work.
If there are any error codes relating to air leaks, be sure to find the source of these leaks and address them.
Replacing faulty fuel injectors can also be a fix for this issue. This depends if your vehicle operates with fuel injectors though.
Sensors should be cleaned and replaced if faulty. The mass airflow and oxygen sensors are the most likely cause.
Any issue that results in the engine acting in an unexpected way is considered very serious. Misfiring can lead to significant damage to the engine and its components.
It is a good idea to have this issue fixed as soon as possible and we rate this as an 8/10 for seriousness.
Popular Fault Codes
These are the most popular fault codes that people are searching for.
Intake Camshaft Position Actuator Circuit / Open (Bank 1)Read about code...
Intake Camshaft Position Timing - Over-Advanced (Bank 1)Read about code...
Intake Camshaft Position Timing - Over-Retarded (Bank 1)Read about code...
Exhaust Camshaft Position Actuator Circuit / Open (Bank 1)Read about code...
Exhaust Camshaft Position Timing - Over-Advanced (Bank 1)Read about code...
Crankshaft Position Camshaft Position Correlation Bank 1 Sensor ARead about code...